This invention relates to an electron beam exposure method and an apparatus therefor in a process preparing a mask for manufacturing of a semiconductor device.
In the case of preparing a mask for manufacturing a semiconductor device, an electron beam exposure apparatus is used to irradiate an electron beam onto a mask blank to conduct an exposure.
An electron beam outputted from an electron gun is irradiated onto a mask blank mounted on a cassette.
However, when an electron beam is irradiated onto the mask blank, it is also irradiated on a cassette portion outside the mask blank to be irradiated. A so called fogging exposure is produced such that the electron beam irradiated onto the cassette is reflected on the upper surface thereof and is reflected on the lower surface of an aperture, and is then excessively irradiated onto the mask blank for a second time. Generally, fogging exposure is hardly produced at the central portion of the mask blank, and fogging exposure is produced to a greater degree as the irradiation position becomes closer to the periphery.
If such a phenomenon takes place, there occur the circumstances equivalent to the case where an electron beam of a fixed dose quantity was not irradiated on the mask blank. Namely, as the irradiation position shifts from the central portion which has not undergone fogging exposure to the peripheral portion, the quantity of irradiation of the electron beam gradually increases. As a result, when the mask blank is developed, there would occur unevenness in the pattern dimension or size which should be originally the same dimension even if the development time is fixed.
The result obtained by measuring such an unevenness in the pattern dimension is shown below. In FIG. 1, a square mask blank having one side of 100 mm is taken as an example, wherein unevenness in an X-direction of the pattern dimension is taken on the ordinate and a position in a direction of diameter corresponding to a wafer on the mask blank is taken on the abscissa. FIG. 2 shows the result obtained by measuring unevenness in a Y-direction of the pattern dimension in the same manner as in FIG. 1.
Furthermore, a position on the mask blank where an unevenness (in X-direction) of the pattern dimension more than 0.18 .mu.m is detected is shown in FIG. 3. This represents the positional relationship in the state when viewed from the upper direction of the mask blank. FIG. 4 shows a position on the mask blank where unevenness more than 0.18 .mu.m in a Y-direction is detected.
As is clear from the measured results mentioned above, as the measurement position shifts from the central portion toward the peripheral portion of the mask blank, unevenness in the dimension increases. Such an unevenness in the pattern dimension leads to a lowering of the manufacturing yield or reliability,